Moiré interferometry is a technique used to study strains and deformations in structural elements with very high accuracy. Moiré is the generic term for full field measurement techniques that utilize the interference effect between some form of specimen grating and a reference grating to magnify the surface deformations and create a contour map that is related to surface displacement—a Moiré fringe pattern or interferogram. For relatively large displacements, mechanical Moiréuses the interference of lined gratings to achieve sensitivity of measurement of up to 25 μm. The state-of-the-art MEMS technology enables measurement sensitivity of up to 5 nm [Horowitz et. al]. Optical Moiré or Moiré interferometry, in which a diffraction grating is illuminated by laser light, increases the sensitivity to sub-micron levels, enabling measurement of elastic strains in engineering materials.
The Moiré pattern is a full field representation of the relative displacement between gratings, be they mechanical or optical. This property of Moiré makes it an excellent tool for observing and quantifying the gradients in localized deformation. In practice, a grating is attached to or generated on, in the case of optical interferometry, the surface of the test piece. The grating deforms together with the test piece and when an undeformed reference grating is superimposed onto it, a moiré pattern depicting the nature and magnitude of the deformation field is obtained. Each Moiré fringe represents a line of constant displacement in the direction perpendicular to the direction of the reference grating. Such devices and methods are used for, among other things, the direct measurement of skin friction.
The development of a floating-element shear stress sensor that permits the direct measurement of skin friction based on geometric Moiré interferometry is reported by Prof. Sheplak and his IMG group at the University of Florida, Horowitz, S., Chen, T., Chandrasekaran, V., Tedjojuwono, K., Nishida, T., Cattafesta, L., and Sheplak, M., “A Micromachined Geometric MoiréInterferometric Floating-Element Shear Stress Sensor,” AIAA Paper 2004-1042, 42nd AIAA Aerospace Sciences Meeting, Reno, Nev., 5-8 Jan., 2004 which is incorporated herein in its entirety for purposes of describing floating-element shear stress sensors of the type in which the invention described herein is particularly useful. As described in detail in the aforementioned publication, this sensor, shown schematically in FIG. 1, is fabricated using an aligned SOI/PYREX® (borosilicate glass) anodic bond, thin-back process that produces aluminum optical gratings on the backside of a 10 mm thick silicon floating element and on the top surface of the Pyrex support wafer.
The device package consists of the sensor die flush-mounted in a Lucite plug with back-side imaging optics and a CCD camera that has an array of 1×1024 pixels. The Moiré fringe pattern is captured using the CCD camera. A least squares fit is then applied to the recorded intensity pattern obtained for a given shear stress input. The least squares procedure fits a sinusoidal intensity pattern to the measured pattern and determines the amplitude, DC offset, frequency and phase of the sinusoid.
As depicted in FIGS. 2 and 3, this procedure is repeated for a given shear stress and the corresponding intensity pattern to obtain the new phase of the shifted sinusoid. From the phase shift, the number of pixels shifted by the Moiré fringe is calculated.
This calibration procedure provides a lookup table to relate shear stress and the phase shift of the Moiré. In addition, using knowledge of the Moiré and optical amplification, the corresponding mechanical displacement of the floating element is computed to give a direct measurement of the wall shear stress.
The fitted data is demonstrated in FIG. 4. The random noise of the data is about 20% of the signal. The error introduced by this noise to the fitting is about 0.13%. This technique is thus very accurate even under noisy signal conditions.
Shortcomings of the above-described prior art system include:
1) for a single sensor, a microscope is required for imaging the CCD array, 1×1024 pixels. In this arrangement, simultaneously probing multiple sensors is possible, however, the arrangement of the CCD array is a major challenge.
2) for a single sensing element, an imaging array is needed to locate the peak intensity of the Moiré. The cost of such an imaging array, including a frame grabber, is approximately $2,000. A special curve fitting chip and electronics add development costs to the current system. These include:                1. the requirement for a uniform light source which impacts the fitting accuracy.        2. the requirement for a microscope for CCD imaging which increases the system size.These and other disadvantages of the prior art system and technique as compared to that of the present invention are presented and discussed in greater detail below.        
Thus, there exist significant and costly shortcomings to the “best” prior art systems currently available for obtaining accurate sub-micron shear stress or direct skin friction measurements. In addition to the cost issues, the prior art systems are: 1) not easily used to further increase the accuracy of measurements as miniaturization increases; 2) not readily useful to perform 2-D area sensing using a single unit sensor; and 3) relatively incapable of performing 2-D sensing with high frequency response.
Alternative imaging techniques, such as pressure sensitive paint (PSP) and temperature sensitive paint (TSP), have been developed for pressure and temperature measurements (see Hubner, J., Carroll, B., Schanze, K., Ji, H., and Holden M., “Temperature- and Pressure-Sensitive Paint Measurements in Short-Duration Hypersonic Flow,” AIAA 99-0388, 37th AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nev., 1999). These technologies are low cost and can take vast amount of data very quickly. The frequency response for those techniques, however, is approximately 1000 Hz. For achieving high frequency response, Miller et. al have developed a 2-D array using a Fabry-Perot interferometry system (see Miller, M. F., Allen, M., Arkilic, E., Breuer, K. S., and Schmidt, M. A., “Fabry-Perot Pressure Sensor Arrays for Imaging Surface Pressure Distributions,”) for pressure measurement. According to a recent survey by Naughton and Sheplak (see Naughton, J. W. and Sheplak, M., “Modern Developments in Shear-Stress Measurement,” Progress in Aerospace Sciences 38 (2002) 515-570, Pergamon) there is no adequate 2-D shear stress sensor that can probe high frequency response flow characteristics.